Following a cancer diagnosis, determining the best course of treatment is of paramount importance. Along with recent advances in understanding the biology and pathways involved in the initiation and progression of certain cancers have come advances in individualizing treatment based on the molecular analyses of these pathways. The most convincing case involves analysis of breast cancer to determine which individuals will most likely require and benefit from adjuvant therapy. Expanding this type of analysis to other cancers holds the promise of similarly impacting cancer therapy. Considering numerous very effective therapies, including cisplatin, induce DNA damage one pathway that is directly related to how individuals respond to certain therapeutic treatments is DNA repair. In the context of cisplatin based cancer chemotherapy, reduced DNA repair capacity is associated with increased sensitivity, while increased repair activity is associated with resistance. The goal of the research described in this application is to develop methodologies to accurately determine DNA repair capacity in cancer tissue, focusing on the nucleotide excision repair (NER) pathway. The NER pathway is also responsible for removing DNA damage resulting from exposure to a variety of insults including cigarette smoke. Our hypothesis is that reduced DNA repair capacity increases the risk of smoking induced carcinogenesis and also contributes to the dramatic initial tumor regression often observed upon administration of cisplatin based therapies for treating lung cancers. The relatively short-lived response and subsequent resistance severely limits the utility of platinum based therapies. Our hypothesis is that the observed resistance is impacted by increased repair in the resistant tumors. To further test these hypotheses an accurate measure of DNA repair activity is required. Measuring gene expression or protein expression, while useful, does not always correlate with bone fide NER repair activity. Numerous NER proteins are regulated not only at the level of mRNA or protein expression, but also by posttranslational modification and protein-protein interactions. Therefore this application focuses on the development of novel methodologies to determine the extent of specific posttranslational modifications of key NER proteins and actual repair activity.